ALTER TABLE Users ADD FirstName NVARCHAR(256) NULL
ALTER TABLE Users ADD LastName NVARCHAR(256) NULL
ALTER TABLE Users ADD Male bit NULL
ALTER TABLE Users ADD BirthDay DATETIME NULL

UPDATE Users SET
FirstName = 'test',
LastName = 'test',
Male = 1,
BirthDay = GETDATE()

ALTER TABLE Users ALTER COLUMN FirstName NVARCHAR(256) NOT NULL
ALTER TABLE Users ALTER COLUMN LastName NVARCHAR(256) NOT NULL
ALTER TABLE Users ALTER COLUMN Male bit NOT NULL
ALTER TABLE Users ALTER COLUMN BirthDay DATETIME NOT NULL
